1. Field of the Invention:
This invention relates to an optical head, and more particularly to an optical head which is suited for use with an optical disk for optically recording information.
2. Description of the Prior Art:
It has heretofore been known to optically record information onto and reproduce recorded information from a thin film made of, for example, a calcogenide glass material, such as a material of Te or a metallic material such as Bi. Since the manner of development and fixation of the material are unnecessary in this case, such applications may be used as are expected apparatuses which require the real time recording of video information, digital information etc. and the playback thereof immediately after the recording, for example, by means of a data recorder, a video disk device of the recording/playback type, etc. In known apparatuses, as a laser source for the recording of information which constitutes an optical head, a large-sized gas laser such as a Ar ion laser, a He-Cd laser and a He-Ne laser is employed, and for the playback of the information, a small-sized He-Ne laser, which is separate from the laser for the recording, is employed in order to avoid mutual interference between the recording and playback signals.
FIG. 1 shows a prior-art video disk device which employs the laser sources of the above-mentioned type. Referring to the figure, numeral 101 designates an Ar laser source for recording information having an output of, for example, 200 mW, while numeral 102 designates a He-Ne laser source for playing back information having an output of, for example, 1 mW. Numeral 103 indicates an optical modulator, numeral 104 a driver for driving the optical modulator to which the input information is applied, numeral 105 designates a lens, numeral 106 a one-half wave plate, numeral 107 a beam splitter, numeral 108 an objective lens, and numeral 109 a disk. Numeral 110 identifies a diffraction grating, numeral 111 a lens, numeral 112 a beam splitter, numeral 113 a galvano-mirror, numeral 114 a cylindrical lens, and numeral 115 a photodetector having a light receiving face which is divided into four areas of equal size. As is well known in the art, auto-focusing based on astigmatism is effected by the lens 114 and the photodetector 115.
The prior-art device above described has various disadvantages as stated below.
(1) The optical systems which are used independently of each other in order to guide laser beams from the respective laser sources onto the identical disk as minute spots of eight become complicated.
(2) In order to avoid mutual interference between the recording laser beam the directions of polarization of and the playback laser beam, the laser beams are caused to intersect orthogonally and are mixed and guided to the identical objective lens by the polarizing beam splitter, as illustrated in the FIG. 1. Therefore, the device becomes expensive, and the laser beams, which cannot be perfectly separated, still tend to mix with each other to some extent causing a deterioration of the focusing accuracy of the system.
(3) To accomplish separation from the recording beam, the playback beam must be used in the condition of the linearly polarized light. For this reason, a polarizing prism cannot be employed as the beam splitter for guiding the beam to the photodetector in the playback optical system. In consequence, a reflected beam from the disk returns to the He-Ne laser source to cause the so-called "back talk phenomenon", with the result that a large noise component appears in a reproduced signal.
(4) The necessary adjustments of the optical systems for focusing the beams from the two independent laser sources into the vicinity of a substantially identical point on the disk as minute spots are very difficult.
(5) In the auto-focusing control means heretofore employed an air floating lens is used which exploits the pneumatostatic pressure. With this means, however, the response is inferior when the vertical movements of the disk occur at a high frequency. Especially in the case where a base plate which constitutes the disk is made of plastic, the air floating lens cannot be used.